Measuring Instruments and Devices MCQ Quiz in मल्याळम - Objective Question with Answer for Measuring Instruments and Devices - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

LVDT (Linear Variable Differential Transformer) is an electromechanical transducer which can turn rectilinear motion into corresponding electric signal.

In LVDT the output signal is AC voltage whose magnitude changes with linear displacement of core within the LVDT coil.

• Thermistor is used for the measurement of temperature
• It is a kind of resistor whose resistivity depends on surrounding temperature
• It is made of the semiconductor material that means their resistance lies between the conductor and the insulator
• The variation in the thermistor resistance shows that either conduction or power dissipation occurs in the thermistor

RTD: 

• RTD stands for Resistance Temperature Detector.
• It has a positive value of the temperature coefficient and is used for measuring temperature.
• Resistance Temperature Detectors (RTDs) are temperature sensors that contain a resistor that changes resistance value as its temperature changes.

• Surface temperature measurement needs the transducer sensor to be in contact with the surface.
• RTD is better than thermocouple in this aspect because the sensor in RTD can be molded into this fine wire or thin-film, while thermocouple is rugged and efficient surface contact is difficult.

• A Wheatstone bridge is a special arrangement of 4 resistors. It can be used to find an unknown resistance.
• If the Wheatstone bridge is balanced, there will be no current flowing through the galvanometer.

• Wheatstone bridge is used to measure the resistance with the help of a comparison method.
• The Wheatstone bridge work on the principle of null deflection.

 

​The bridge is balanced when:

\(\frac{P}{R}=\frac{Q}{S}\)

∴ A Wheatstone bridge is balanced if the ratio of resistors on one side of the bridge equals to the ratio of resistors on the other side.

• Thermocouple has a constant temperature at one end and is used to measure temperature.
• Thermistor has a negative temperature coefficient, i.e. when the temperature increases the resistance will decrease.
• A strain gauge is a sensor whose resistance varies with applied force. It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured.

• RTD stands for Resistance Temperature Detector. It has a positive value of the temperature coefficient.

Concept:

We can extend the range of ammeter by keeping a shunt resistance as shown:

Rm = internal resistance of the coil

Rsh = Shunt resistance

I = Required full-scale range

Im = Full scale deflection of current

As the two resistances, Rm and Rsh are in parallel, the voltage drop across the resistance will be equal.

\({I_m}{R_m} = \left( {I - {I_m}} \right){R_{sh}}\)

\({R_m} = \left( {\frac{I}{{{I_m}}} - 1} \right){R_{sh}}\)

\({R_{sh}} = \frac{{{R_m}}}{{\left( {\frac{I}{{{I_m}}} - 1} \right)}}\)

\({R_{sh}} = \frac{{{R_m}}}{{\left( {m - 1} \right)}}\)

Where \(m = \frac{I}{{{I_m}}}\)

‘m’ is called multiplying power

Calculation:

Given:

Meter resistance (Rm) = 500 Ω

Full scale deflection current (Im) = 25 μA

Required full scale reading (I) = 10 A

∴ The required shunt resistance that should be connected in parallel/shunt will be:

\({R_{sh}} = \frac{{{R_m}}}{{\left[ {\frac{I}{{{I_m}}} - 1} \right]}}\)

\({R_{sh}} = \frac{{500}}{{\left( {\frac{{10}}{{25\mu}} - 1} \right)}} = 1.25\;m\Omega{\rm{ }}\)

• To increase the ranges of ammeter, we need to connect a small shunt resistance in parallel with ammeters.
• To increase the ranges of a voltmeter, we need to connect a high series of multiplier resistance in series with voltmeters.

• The instruments which use the heating or thermal effect of the current for knowing their magnitude such type of instrument is known as the hot wire instrument.
• The hot wire instrument is used for both the AC and DC current
• Hot wire instrument works on the principle that the length of the wire increases because of the heating effect of the current flow through it.
• When the current is passed through the fine platinum-iridium wire it gets heated up and expands.
• The sag of the wire is magnified, and the expansion is taken up by the spring
• This expansion causes the pointer to deflect, indicating the value of the current
• This expansion is directly proportional to the heating effect of the current and hence directly proportional to the square of the RMS value of the current.
• Therefore, the meter may be calibrated to read the rms value of the current.​

• A multimeter is a device that measures voltage, current, and resistance.
• A multimeter always reads the RMS value of the voltage of an AC source.
• The peak value of voltage in an AC source is the highest voltage that the waveform will reach like the peak is the highest point on a mountain.
• The RMS (Root-Mean-Square) value of voltage is the effective value of the total waveform of an AC voltage.
• It is taken as equal to the DC signal that would provide the same average power as the periodic signal.

Mathematically RMS value of voltage is given by:

Vrms = V0 / √ 2 

Where Vrms is RMS voltage and V0 is maximum voltage.

The pressure measurement using bourden tube and LVDT is shown in below figure

The pressure applied to bourden tube is converted into displacement which moves the LDVT core and the final measurement of pressure is formed.

In moving coil type instruments the current is fed into the coil which is connected to a pointer showing measurement.

When current is passed in the coil a magnetic field is generated, the generated field interacts with the magnetic field of the permanent magnet and a deflecting torque is produced proportional to the current fed into the coil. This is the basic measuring principle used in moving coil instruments.